This invention relates to a sintered alloy used in a contact forming material for a vacuum interrupter, a vacuum circuit breaker or a vacuum circuit interrupter, and, more particularly, to a contact forming material for a vacuum interrupter having an improved current chopping characteristic and high-frequency arc-extinguishing characteristic.
Contacts for a vacuum interrupter for carrying out current interruption in a high vacuum utilizing an arc diffusion property in a vacuum, are constituted of two opposing contacts, i.e., stationary and movable contacts. When the current of an inductive circuit such as a motor load is interrupted by means of the vacuum interrupter, an excessive abnormal surge voltage is generated and a load instrument tends to be broken.
The reasons why such an abnormal surge voltage is generated are attributable to phenomena such as chopping phenomenon generated when a small current is interrupted in a vacuum (a current interruption is forcedly carried out before the waveform of an alternating current reaches the natural zero point) and a high-frequency arc-extinguishing phenomenon.
The value Vs of the abnormal surge voltage due to the chopping phenomenon is expressed by a product of the surge impedance Zo of a circuit and the current chopping value Ic, i.e., Vs=Zo.Ic. Accordingly, in order to reduce the abnormal surge voltage Vs, the current chopping value Ic must be decreased.
In order to meet the requirements described above, there have been developed vacuum swtiches wherein contacts composed of tungsten carbide (WC)-silver (Ag) alloys are used (Japanese patent application No. 68447/1967 and U.S. Pat. No. 3,683,138). Such vacuum switches have been put to practical use.
The contacts composed of such Ag-WC alloys have the following advantages:
(1) the presence of WC facilitates electron emission;
(2) the evaporation of the contact forming material is accelerated by heating of the surface of electrodes due to collision of field emission electrons; and
(3) the contacts exhibit a low chopping current characteristic which is excellent, e.g., for remaining an arc by decomposing a carbide of the contact forming material by the arc and forming a charged particle.
Another contact forming material exhibiting a low chopping current characteristic is a bismuth (Bi)-copper (Cu) alloy. Such a material has been put to practical use to form a vacuum interrupter (Japanese Patent Publication No. 14974/1960, U.S. Pat. No. 2,975,256, Japanese Patent Publication No. 12131/1966 and U.S. Pat. No. 3,246,979). Of these alloys, those containing 10% by weight (hereinafter referred to as wt. %) of Bi (Japanese Patent Publication No. 14974/1960) have suitable vapor pressure characteristics and therefore exhibit low chopping current characteristics. Those containing 0.5 wt. % of Bi (Japanese Patent Publication No. 12131/1966) segregate Bi in crystal boundaries and this therefore renders the alloy per se brittle. Thus, a low welding opening force is realized and the alloys have an excellent large current interruption property.
Another contact forming material exhibiting a low chopping current characteristic is an Ag-Cu-WC alloy wherein the ratio of Ag to Cu is approximately 7:3 (Japanese patent application No. 39851/1982). In this alloy, a ratio of Ag to Cu which has not been used in the prior art is selected and therefore it is said that stable chopping current characteristic is obtained.
Furthermore, Japanese patent application No. 216648/1985 suggests that the grain size of an arc-proofing material (e.g., the grain size of WC) of from 0.2 to 1 micrometer is effective for improving the low chopping current characteristic.
A low surge property is required for vacuum breakers, and therefore a low chopping current characteristic (low chopping characteristic) has been required in the prior art.
In recent years, vacuum interrupters have been increasingly applied to inductive circuits such as motors, and high surge impedance load. Accordingly, vacuum interrupters must combine an even more stable low chopping current characteristic and a satisfactory high-frequency arc-extinguishing characteristic (high-frequency current interruption capability). This is because it has turned out that surges due to multiple reignitions are undesirable for insulation of the load, as well as for surges due to current chopping.
Heretofore, there have been no contact forming materials which simultaneously satisfy these two characteristics.
That is, while a surge due to the current chopping described above (overvoltage) can be improved by reducing a current chopping value, a surge due to repeated high-frequency reignition is one wherein a recovery voltage value is increased by interrupting a high-frequency current which passes depending upon the circuit conditions when a dielectric breakdown is generated between electrodes after current chopping, and furthermore a recovery voltage value is increased by repeating a process in which a dielectric breakdown is generated between the electrodes, whereby an excessively large surge voltage is generated. In this case, a surge is generated in order to extinguish a high-frequency current, and the generated surge can be reduced by improving the high-frequency arc-extinguishing characteristic so that the surge voltage is reduced. Therefore, it is necessary to improve and stabilize the arc reestablishment characteristic of a high-frequency current discharge.
In the contacts composed of the WC-Ag alloys (Japanese patent application No. 68447/1967 and U.S. Pat. No. 3,683,138), the chopping current value per se is insufficient, and no regard is paid to the improvement of high-frequency arc-extinguishing characteristic.
In the 10 wt. % Bi-Cu alloys (Japanese Patent Publication No. 14974/1960 and U.S. Pat. No. 2,975,256) the amount of a metal vapor fed to the space between the electrodes is reduced as the number of make and break increases. The deterioration of low chopping current characteristic occurs and the deterioration of withstand voltage occurs depending upon the amount of an element having a high vapor pressure. Furthermore, the high-frequency arc-extinguishing characteristic is not entirely satisfactory.
In the 0.5 wt. % Bi-Cu alloy (Japanese Patent Publication No. 12131/1966 and U.S. Pat. No. 3,246,979), its low chopping current characteristic is insufficient.
In the Ag-Cu-WC alloys wherein the weight ratio of Ag to Cu is approximately 7:3 (Japanese patent application No. 39851/1982) and the alloys wherein the grain size of the arc-proofing material is from 0.2 to 1 micrometer (Japanese patent application No. 216648/1985), their high-frequency arc-extinguishing characteristic is not entirely satisfactory.
An object of the present invention is to provide a contact forming material which combines an excellent low chopping current characteristic and high-frequency arc-extinguishing characteristic and which meets the requirement for a vacuum breaker to be used under severe conditions.